Solar charger (simple)

This is a simple solar charger circuit that can be used to charge up NiMH batteries. It contains no mechanism for current regulation and has not been thoroughly tested, so it should be used with caution. The method is basically a retranscription of the following article.

Ingredients

• 1 x Schottky diode (0.2V, best) or alternatively Germanium diode (0.3V)
• 1 x solar cell
• NiMH batteries

Total batteries voltage should be lower that the solar panel's maximum voltage but ideally there should never be more than a 5-7V difference. In a typical application I would use a 4-5V solar panel with 3 x 1.2V batteries = 3.6 V.

Note: Most of the circuits described in Accrochages use low-voltage circuits that can run with 3-4 V. The needed number of batteries and solar cell's voltage must be adapted to the circuit.

Instructions

Charge the NiMH batteries once with a charger, especially if they have never been used or have not been used in a long time.
The circuit is very simple and is described in the schematics in attachment to this page. The solar cell and the batteries are plugged in parallel into the circuit that needs to be fed. The diode avoids the current from flowing back to the solar cell during nighttime.
A resistor can be added between the solar cell and the batteries to avoid the solar cell from damaging the batteries (optional, has not been tested).

References

• Solar battery charging
• Same with a LM317T regulator
• Read the comments following this post regarding overcharging issues and solutions
• This post describes a Coffee table project using solar--powered Arduino
• Instructables showing the exact same circuit (they also add a 100uF cap to prevent problems due to fluctuating current)